The present invention relates generally to the field of diagnostic radiography and, more particularly, to an anti-scatter grid capable of yielding high resolution, high contrast radiographic images and a laser fabrication method.
During medical diagnostic radiography processes, x-radiation impinges upon a patient. Some of the x-radiation becomes absorbed by the patient's body, and the remainder of the x-radiation penetrates through the body. The differential absorption of the x-radiation permits the formation of a radiographic image on an image receptor such as a photosensitive film, an image intensifier, or a digital receptor.
Of the x-rays that pass through the body, primary radiation travels unimpeded and directly along the path from which the x-rays were originally emitted from the source. Scattered radiation is that which passes through the body, is scattered by the body elements, and thus travels at an angle from the original path. Both primary and scattered radiation will contribute to an image, but scattered radiation, by nature of its trajectory, reduces image contrast (sharpness) of the projected image. In conventional posterior/anterior chest x-ray examinations, for example, about sixty percent of the radiation that penetrates through the body can be in the form of scattered radiation and thus impart a significant loss of image contrast. Therefore, it is desirable to filter out as much of the scattered radiation as possible.
One embodiment for filtering scattered radiation includes an anti-scatter grid which is interposed between the body and the image receptor. Scattered radiation impinges upon absorbent (opaque) material in the grid and becomes absorbed. Also absorbed by the absorbing material, however, is a portion of the primary radiation. The radiographic imaging arrangement of this embodiment provides higher contrast radiographs by virtue of the significant reduction of the scattered radiation, but necessitates an increase in radiation dosage to the patient in order to properly expose the photographic element. The increased radiation requirement results in part because the scattered radiation no longer constitutes part of the imaging x-ray beam, and in part because as much as thirty percent or more of the primary beam impinges upon the absorbing material in the grid and itself becomes filtered out (i.e. absorbed).
The increased radiation required for the exposure can be a factor of five (5) or more, i.e., the patient can receive five times the x-radiation dose when the grid is used as a part of the radiographic system. Because high doses of x-radiation pose a health hazard to the exposed individual, there has been a continual need to reduce the amount of x-radiation a patient receives during the course of a radiographic examination.
Many conventional grids use thin lead strips as the x-ray absorber and either aluminum or fiber composite strips as transparent interspace material. Conventional manufacturing processes consist of laminating individual strips of the absorber material and non-absorber interspace material by gluing together alternate layers of the strips until thousands of such alternating layers comprise a stack. Furthermore, to fabricate a focused grid, the individual layers must be placed in a precise manner so as to position them at a slight angle to each other such that each layer is fixedly focused to a convergent line: the x-ray source. After the composite of strips is assembled into a stack, it must them be cut and carefully machined along its major faces to the required grid thickness that may be as thin as 0.5 millimeters, the fragile composite then being, for example, 40 centimeters by 40 centimeters by 0.5 millimeters in dimension and very difficult to handle. The composite then must further be laminated with sufficiently strong materials so as to reinforce the fragile grid assembly and provide enough mechanical strength for use in the field. Accidental banging, bending, or dropping of such grids can cause internal damage, i.e., delamination of the layers which cannot be repaired, rendering the grid completely useless.
Due to the nature of the stacking process, grids fabricated from a conventional stack of layers of x-ray absorbers and transparent interspace materials are limited to linear geometries. Furthermore, even if the absorbers and the interspace materials are kept within specification ranges, the process often creates a cumulative line positioning error consisting of the sum of the layers' thickness variations and the thickness variations of adhesives between the layers.
A significant parameter in the grid design is the grid ratio, which is defined as the ratio between the height of the x-ray absorbing strips and the distance between them. The ratios typically range from 4:1 to 16:1. Because a value of about 0.050 millimeter lead thickness is a practical limit imposed by current manufacturing limitations, i.e., it being extremely difficult to handle strips at this thickness or thinner, a grid with a ratio of 4:1 with a line rate of 60 lines per centimeter demands that the interspace material be 0.12 millimeters in width and results in a grid that is only 0.028 millimeters thick. Because of the manufacturing limitations, the lead strips in these grids are generally too wide and, consequently, yield a large cross-sectional area that undesirably absorbs as much as thirty percent or more of the primary radiation. Furthermore, the thick strips result in an undesirable shadow-image cast onto the image receptor. To obliterate the shadows, it becomes necessary to provide a mechanical means for moving the grid during the exposure period. This motion of the grid causes lateral decentering and can consequently result in absorption of an additional twenty percent of the primary radiation. Thus the use of wide absorber strips requires a significant increase in patient dosage to compensate this drawback.
A present goal of the electronic imaging industry is to replace film-based imaging systems. Image detectors such as charge coupled device (CCD) detectors and flat panel amorphous silicon (.alpha.-Si) detectors are likely to be used in the future as a substitute for films and electronic tubes. Such image detectors include large arrays of elements with pixel pitches of 200 micrometers or less. Conventional x-ray grids cannot be optimized with these arrays because they are fabricated with straight lines and generally cannot match the array pitch. If absorbing material of the x-ray grids overlaps the active areas of the image detectors, the efficiency of the image detectors is reduced and Moire patterns can be generated.
Commonly assigned U.S. Pat. No. 5,557,650, issued Sep. 17, 1996, discloses a method for fabricating an anti-scatter x-ray grid which includes providing a substrate having channels therein and material that is substantially non-absorbent of x-radiation, and filling the channels with absorbing material that is substantially absorbent of x-radiation. In one embodiment, the substrate is provided by sawing a plastic substrate with a thin circular blade and the channels are filled by melting absorbing material and flowing the melted absorbing material into the channels. These grids have increased resolution over prior lamination techniques.